Generating Electricity From a Heating Boiler
by TreeHugger 
on 05.12.05
Powergen, a utility company in the U. K., says it has invented a central heating boiler which generates electricity for households. Powergen is looking for hundreds of customers prepared to buy these personal power stations in a limited launch this winter. When the electricity isn't being used it will be fed back to the grid. The boiler fits under the kitchen counter. It makes a slight humming noise and has an LCD display showing the temperature of the heated water and that its generating 850 watts of electricity. The boiler is based on the Stirling engine, created up by the Scottish inventor Robert Stirling in 1816.
There's a gas burner at the top. It heats up four cylinders, each of which contains nitrogen gas and a piston. The gas expands as it warms up, pushing the pistons down. The pistons are cooled on the central heating water, which passes underneath, so they go up again. The resulting rotation is 1,600 times a minute, turning a generator and producing a constant supply of hot water. It's known as an external combustion engine. The technical name for this particular application is "Micro Combined Heat and Power" or "Micro CHP". The drawback is the cost of the boilers and installation, which currently costs between £2,500 ($4,660) and £3,000 ($5,593). Powergen is hoping that the cost will go down if the system becomes popular. :: Powergen via BBC News [by Justin Thomas]
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How energy effecient would it be in comparison to, say, a conventional coal power plant?
As a homeowner with radiators, I'm interested in anything that produces both hot water and electricity. Since some/many houses now come with radiant floor heat, perhaps they need to expand the use, making it relatively less expensive. I hate to think what it will cost me to change my boiler out. Probably at least $5000US.
Several firms also make micro turbines. Plug Power makes fuel cell based CHP units for sale in Japan. So far these are targeted to light commerrcial applications where the heat and power are continuously put to good use, unlike a home where there will be times when it does not: like on vacation when you need to keep the heat on to prevent pipes freezing:i.e you'd need to have a grid tie in for selling electricity back or see the total fuel efficiency drop dramatically.
This was created in New Zealand, not the UK, and commercialised from University of Canterbury research work.
http://www.whispertech.co.nz/main/WTNews/
http://whispertech.co.nz/main/contact/
http://www.caenz.com/info/publications/newsletters/downloads/IB29.pdf
cheers
I always thought the BIG idea behind the sterling engine was that once it got running, it was self propelled. I think that has been missed or left out here. Wouldn't it be great if this little engine could get started with its suggested natural gas power but then run itself off the electricity that it produces. Now that would be super.
see also stuff like http://www.jxcrystals.com/TPV/RomeABB.pdf
for thermophotvoltaic heater/generators, where radiant energy from the combustion (far below the visible spectrum, in the infrared) is converted by to electricity by photovoltaic cells with a smaller band gap than solar cells, clustered around the burner. I have no idea which heater/generator approach is most efficient.
Kyle, I think you're thinking of "Perpetual Motion" machines, which are physically impossible. (if you'll pardon the pun)
Also note that the "it" in the sentence "it's known as an external combustion engine" refers to this particular implementation of a Stirling engine. Any source of heat (differential) can power a Stirling engine, from wood to solar to geothermal to body-heat.
How well a specific Stirling engine works with a heat source depends on it's design. An engine designed for gas burning probably won't be well-suited/tuned for a solar source.
The most efficient way to produce hot water is probably an "on demand water heater", though.
Here's an example:
http://www.gotankless.com/
I wonder what kind of power output you could get if you set up an array of mirrors on your roof to focus sunlight onto the "hot spot" of the boiler. Or maybe fresnel lenses. Obviously this would only give you hot water during the day.
whirlyman, Issue 42 of Permaculture Magazine (see http://www.permaculture.co.uk/mag/PCM_Back_Issues.html if you can't find a local copy) had one article which mentioned just this sort of setup. Instead of water though, they used vegetable oil as the heat transfer fluid to get higher temperatures, and store it through the night. I'm at work so I don't have the issue handy to give any more details.
Somewhat related:
"Sun catchers tuned to crank out the juice"
http://eetimes.com/at/news/showArticle.jhtml?articleID=53700939
Mikhal
Biolers are not just for hot water for the tap. Boliers are generally used for heating as well. Instant hot waer heaters are not intended for heating.
There is also some good info here about efficient water heating:
http://www.tanklessexperts.com/index.php
Almost four years ago when I started building my new house, some companies had been written up in UK newspapers of having developed Stirling boilers for home use and I contacted them. The comments then were that basically they only now had to discuss which colour the boiler should have and that it would be on the market very shortly - which unfortunately it never materialised in time for my house. When will some/any company market sterling boilers for real? It would come in handy for the next house I hope to build in two or three years time.